Coaxial two port waveguide flowline sensor

Abstract

A two-port, coaxial, waveguide flowline sensor for transmitting TEM mode electromagnetic radio frequency signals through borehole fluids flowing through a longitudinal flowline cavity of the waveguide. The waveguide flowline sensor responds to changes in the conductivity, dielectric constant, and state of the fluid at high borehole temperatures and pressures. The sensitivity of the sensor is enhanced by impedance mismatches between two feedthrough waveguides comprising the ports of the sensor, and a flowline cavity waveguide.

What is claimed is: 
     
       1. A two-port, coaxial, waveguide flowline sensor for a borehole formation tester engaged in the real time testing of a formation while in a borehole, which comprises: a top subassembly having a first cavity in a first lower surface for creating an upper surface of a waveguide flowline cavity through which a formation fluid may flow, and having two spaced apart vertical tunnels leading from a first top surface of said top subassembly to said first cavity;   a pair of conductors each seated in one of said two spaced apart vertical tunnels and extending above said first top surface and below said first lower surface within said first cavity;   a pair of electrical insulators each seated in one of said two spaced apart vertical tunnels and each surrounding one of said pair of conductors to form feedthrough waveguides which effect a high pressure seal;   a pair of coaxial connectors each connected to one of said pair of conductors and fastened to said first top surface for creating an input electrical port and an output electrical port to said first cavity;   a central conductor physically and electrically connected to said pair of conductors and spaced coaxially within inner walls of said waveguide flowline cavity to form a coaxial flowline waveguide through which a TEM mode RF signal may propagate, wherein said coaxial flowline waveguide is impedance mismatched with said feedthrough waveguides; and   a bottom subassembly having a second cavity in a second top surface of said bottom subassembly for forming said waveguide flowline cavity with said first cavity when said bottom subassembly is sealingly attached to said top subassembly, said waveguide flowline cavity enclosing said central conductor coaxially about a longitudinal axis in a spaced apart relation.   
     
     
       2. The waveguide flowline sensor of claim 1, wherein said TEM mode RF signal is chosen in concert with physical dimensions of said waveguide flowline cavity, said pair of conductors, said pair of electrical insulators, and said central conductor, which are selected in accordance with characteristic impedances Z N  of said feedthrough waveguides and said coaxial flowline waveguide to maximize the rate of change of the reciprocal of S 22  for a change in characteristic impedance of said coaxial flowline waveguide resulting from a change in complex permittivity of said formation fluid at an operating frequency of said TEM mode RF signal. 
     
     
       3. The waveguide flowline sensor of claim 1, wherein said bottom subassembly has a third tunnel leading from said waveguide flowline cavity to an outer surface of said bottom subassembly for channeling flow of said formation fluid into said waveguide flowline cavity, and said waveguide flowline cavity is oblong and cylindrical in shape and has an annular fluid flow outlet at one end and a tunnel opening near an opposite end leading to said third tunnel through which said formation fluid enters said waveguide flowline cavity. 
     
     
       4. The waveguide flowline sensor of claim 3, wherein said central conductor is oblong and cylindrical in shape, and coaxially aligned along said longitudinal axis and suspended within said waveguide flowline cavity by said pair of conductors for receiving an RF signal on one of said pair of conductors and causing said RF signal to propagate as a TEM mode wave within said waveguide flowline cavity to a second of said pair of conductors. 
     
     
       5. A waveguide flowline sensor for detecting small changes in a complex permittivity of a formation fluid by enhancing phase delay and attenuation of an RF signal propagating through said waveguide flowline sensor, which comprises: conductor means including a pair of feedthrough waveguides for providing an electrical path resistive to high differential pressure from an outer surface of said waveguide flowline sensor to an inner cavity of said waveguide flowline sensor, said inner cavity having a fluid flow inlet evolving from a tunnel at one end and a fluid flow outlet at another end for channeling said formation fluid through said inner cavity; and   a central conductor located within said inner cavity, in electrical communication with said conductor means, and coaxially aligned along a longitudinal axis of said inner cavity but spaced apart from inner walls of said inner cavity, for forming a coaxial flowline waveguide with said inner walls, wherein said coaxial flowline waveguide is impedance mismatched with said feedthrough waveguides in accordance with a complex value S 22  for enhanced sensitivity to changes in said complex permittivity, and said RF signal propagates through said coaxial flowline waveguide in a TEM mode.   
     
     
       6. The waveguide flowline sensor of claim 5, wherein an operating frequency of said RF signal is selected in concert with physical dimensions of said feedthrough waveguides, said coaxial flowline waveguide, and said central conductor, with said physical dimensions being selected in accordance with characteristic impedances Z N  of said feedthrough waveguides and said coaxial flowline waveguide to maximize the rate of change of the reciprocal of S 22  for a change in characteristic impedance of said coaxial flowline waveguide resulting from a change in said complex permittivity of said formation fluid at said operating frequency. 
     
     
       7. The waveguide flowline sensor of claim 6 wherein said physical dimensions transverse to direction of propagation of said RF signal are less than one-half wavelength of said RF signal to restrict mode of propagation of said RF signal to said TEM mode. 
     
     
       8. The waveguide flowline sensor of claim 5, wherein said pair of feedthrough waveguides are spaced apart a distance which is less than one inch, and said operating frequency is less than 500 MHz. 
     
     
       9. The waveguide flowline sensor of claim 5, wherein an absolute measurement is used to detect small changes in said complex permittivity. 
     
     
       10. The waveguide flowline sensor of claim 5, wherein said waveguide flowline sensor is made of Inconel 725 stainless steel. 
     
     
       11. The waveguide flowline sensor of claim 5, wherein said RF signal may be monochromatic, modulated, swept over a broad bandwidth, or have high harmonic content.